The present invention relates to a ferroelectric film characteristic measuring instrument for measuring the characteristics of a ferroelectric film, particularly the electrical characteristics of a ferroelectric capacitor formed by using a ferroelectric film, a measuring method of the same, and a measuring method for measuring a semiconductor memory device by applying the measuring method of the characteristics of the ferroelectric film.
In recent years, semiconductor memory devices using ferroelectric film have come under attention as ideal memories since they enable access at write and read speeds comparable to those of a DRAM (dynamic random access memory), enable realization of a number of rewrites greater than that of an EEPROM (electrically erasable and programmable read only memory), and do not lose the written information even when power is not supplied, that is, enable so-called non-volatile storage. For this reason, there has been active research into the electrical characteristic of ferroelectric film in the past few years.
When applying an electric field to a ferroelectric thin film, the phenomenon of electrical polarization where a charge is induced at the surface of the ferroelectric crystal takes place. When the direction of the input electric field is inverted, the direction of the polarization also inverts, that is, polarization inversion takes place. Further, there is a characteristic absent in ordinary dielectrics that the polarization of the ferroelectric thin film remains even if the electric field disappears. A memory which stores digital information of xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d according to the residual polarization of the ferroelectric film has already been realized. Generally, this kind of memory is called a ferroelectric random access memory (FRAM).
In the development of a ferroelectric memory, it is important to obtain an accurate grasp of the characteristics of the ferroelectric materials. For example, since the reliability, the switching time of the polarization inversion, the residual polarization, and other characteristics of the ferroelectric material directly influence the characteristic of the memory, it is important to accurately measure these characteristics of the ferroelectric material to form a semiconductor memory with a high reliability.
Up to now, different kinds of methods have been proposed for measuring the characteristics of a ferroelectric material. One enables measurement of the characteristic of the polarization inversion of the ferroelectric material by measuring the transient current of a capacitor formed by sandwiching a ferroelectric film with two electrodes laid facing each other when applying a pulse signal having a predetermined amplitude to the electrode of the capacitor.
FIG. 4 shows an example of the measuring instrument for measuring the characteristics of a ferroelectric capacitor. As illustrated, the measuring instrument is constituted by a pulse generator 10, a signal transmission line 20, a resistor 30, and an amplifier (preamplifier) 40. The capacitor CFE is the ferroelectric capacitor to be measured.
In the illustrated measuring instrument, a pulse signal having alternating positive and negative amplitudes (below, referred to as a bipolar pulse) is generated by the pulse generator 10 and applied to the ferroelectric capacitor CFE through a transmission line 20. At the ferroelectric capacitor CFE, an electric field in accordance with the applied pulse voltage is generated and polarization occurs accordingly. A current iFE accompanying polarization inversion flows to the capacitor CFE since polarization inversion of the ferroelectric occurs each time a pulse is applied.
FIGS. 5A and 5B show the waveform of the bipolar pulse applied to the ferroelectric capacitor CFE and the waveform of the transient current iFE of the capacitor CFE when the pulse is applied. The bipolar pulse shown in FIG. 5A is generated by the pulse generator 10 and is applied to the ferroelectric capacitor CFE through the transmission line 20. The transient current iFE shown in FIG. 5B is generated in the ferroelectric capacitor CFE in accordance with this.
Since the current iFE is extremely small and cannot be measured as it is, it is amplified for measurement. In the measuring instrument, the small current iFE is input to the resistor 30 to generates a voltage drop Vi. The voltage Vi is input to the preamplifier 40, whereby a voltage Vo amplified by the preamplifier 40 is output. The output voltage Vo of the preamplifier 40 can be further amplified if necessary. In this way, with the ferroelectric capacitor measuring instrument of the present example, the current iFE accompanying the polarization inversion of the ferroelectric capacitor CFE to be measured is converted to the voltage signal Vi and amplified, so the voltage signal Vo with a large amplitude that is able to be observed or measured can be obtained. By measuring the output voltage Vo of the preamplifier 40, the transient current at the time of polarization inversion of the ferroelectric capacitor CFE can be measured and the electrical characteristics of the ferroelectric capacitor CFE can be studied. Particularly, by measuring the transient current iFE of the ferroelectric capacitor CFE at the time of repeated polarization inversion, the fatigue life of the ferroelectric material can be estimated, so this measuring method is an important means for estimating the reliability of the ferroelectric memory.
But along with the increasing capacity and density of semiconductor memories, the dimensions of the ferroelectric thin films used in non-volatile memory devices are becoming smaller. For example, in the recently produced non-volatile memory having a storage capacity of 4 Mb (megabytes), the dimensions of the ferroelectric thin film used in the ferroelectric capacitor are on the submicron order, that is, are smaller than 1 xcexcm (micrometer). With such a small ferroelectric capacitor, when applying a pulse signal having a predetermined amplitude to the electrodes, the polarization current generated along with the polarization inversion of the ferroelectric is extremely weak and measurement of the polarization current becomes difficult.
Further, in the above ferroelectric capacitor measuring instrument, along with the decrease of the capacity of the ferroelectric capacitor CFE being measured, the effect of the parasitic capacity of the transmission line 20 can no longer be ignored. In this case, since the current due to the charging and discharging of the parasitic capacity is sufficiently larger than the current due to the polarization inversion of the ferroelectric material, there is the demerit that the current iFE generated at the time of polarization inversion of the ferroelectric material cannot be accurately measured based on the output voltage Vo of the measuring instrument.
The present invention was made in consideration with such a circumstance and has as an object thereof to provide a ferroelectric film characteristic measuring instrument which is able to measure the transient current of a ferroelectric capacitor having very small capacity at the time of polarization inversion with a high precision and a measuring method of the same.
To attain the above object, the ferroelectric film characteristic measuring instrument of the present invention is a ferroelectric film characteristic measuring instrument for measuring the characteristics of a ferroelectric film sandwiched between first and second electrodes arranged facing each other, comprising a pulse generator for generating a pulse signal to be input to the ferroelectric film, a conductive probe having a pointed tip for contacting said first electrode and inputting said pulse signal to said first electrode, and a current measuring circuit connected to said second electrode and measuring a current flowing in said ferroelectric film at the time of application of the pulse signal as an output current of said second electrode.
Further, in the present invention, preferably the pulse generator generates a first pulse signal having an alternating positive and negative amplitude and a second pulse signal having only a positive or negative amplitude. Further, the above pulse generator generates pulse signals having a positive amplitude and a negative amplitude twice each in succession. Further, the probe is a tip that is used in a scanning force microscope (SFM) or a scanning tunneling microscope (STM).
Further, the present invention further comprises a processing circuit for calculating the difference between the current measured when said first positive pulse is input and the current measured when said second positive pulse is input. Further, the processing circuit calculates the difference between the current measured when said first negative pulse is input and the current measured when said second negative pulse is input.
Further, the method for measuring the characteristics of a ferroelectric film of the present invention is a ferroelectric film measuring method which measures the characteristics of the ferroelectric film by measuring the current flowing through a ferroelectric film when sandwiching said ferroelectric film between said first and second electrodes and applying predetermined signals to the first and second electrodes, comprising a first step of bringing a thin probe formed by a conductor into contact with said first electrode and applying a pulse signal to said first electrode through said probe, a second step of measuring an output current of said second electrode when applying said pulse signal, and a third step of measuring the characteristics of said ferroelectric film in accordance with the measured current.
Further, in the present invention, preferably, said first step comprises applying a first pulse signal having alternating positive and negative amplitudes and a second pulse signal having only a positive or negative amplitude to said first electrode, said second step comprises measuring said output current when said first and second pulse signals are applied, and said third step comprises measuring the characteristics of the ferroelectric film in accordance with the measured currents when said first and second pulse signals are applied.
Alternatively, in the present invention, preferably, said first step comprises applying pulses having a positive amplitude and a negative amplitude twice each in succession to said first electrode, said second step comprises measuring output currents when the first positive or negative pulse and second positive or negative pulse are applied, and said third step comprises finding the difference between the measured currents when applying the first and second pulses to measure the characteristics of the ferroelectric film in accordance with the difference of the currents.
Further, the method for measuring a semiconductor memory device of the present invention is a method for measuring a semiconductor memory device including a ferroelectric film comprising a first step of selecting any one semiconductor memory device from a plurality of semiconductor memory devices, a second step of sandwiching said ferroelectric film of the selected semiconductor memory device between first and second electrodes, a third step of bringing a thin probe formed by a conductor into contact with said first electrode and applying a pulse signal to said first electrode through said probe, a fourth step of measuring an output current of said second electrode when applying said pulse signal, and a fifth step of measuring the characteristics of said ferroelectric film in accordance with said measured current and measuring the semiconductor memory device in accordance with the results of the measurement.
According to the present invention, it is possible to measure the characteristics of the ferroelectric capacitor by measuring the output current of a second electrode when applying a pulse signal to a first electrode using a thin probe formed by a conductor in a ferroelectric capacitor formed by sandwiching a ferroelectric film with two electrodes arranged facing each other. The pulse signal applied to the first electrode is for example a bipolar pulse having alternately positive and negative amplitudes or a unipolar pulse having only a positive or negative amplitude. It is possible to measure the output currents when applying such pulse signals and measure the transient current generated by the polarization inversion of the ferroelectric. The characteristics of the ferroelectric capacitor are measured based on the results of the measurement. Further, a so-called double pulse generating positive and negative amplitudes twice each in succession is generated and input to the first electrode. By finding the difference of the currents measured when the first and second pulses are applied, the influence of the charge current of the parasitic capacity of the ferroelectric capacitor and the transmission line etc can be removed and only the polarization current accompanying the polarization inversion of the ferroelectric can be measured and therefore the characteristics of the ferroelectric can be accurately measured.